1. Field of the Invention
This invention relates generally to an apparatus for transmitting rotary power in a vehicle powertrain. More particularly it pertains to full-time and part-time all-wheel-drive (AWD) systems driven from a transmission with a planetary final drive.
2. Description of the Prior Art
Power takeoff unit efficiency is a concern in AWD vehicles derived from front wheel drive vehicle platforms, partially due to kinematics dictated by the speed and torque of the transmission output.
The final drive unit of a transmission normally increases torque by a factor of approximately 4.0-4.5 before transmission output torque is delivered to the input of a differential mechanism, which drives the primary wheels of the vehicle, i.e., the front wheels. Then, the amplified torque is stepped down before it is delivered to a power takeoff unit (PTU), which drives the secondary wheels, i.e., the rear wheels.
In a powertrain for a motor vehicle whose transmission includes a planetary final drive unit that amplifies torque, it is possible to split the transmission output torque between the front wheels and rear wheels before performing the planetary torque amplification.
It is also possible in a part-time or torque-on-demand system to deliver the transmission output torque to the power takeoff unit directly, thereby bypassing the torque amplification that normally would occur in the final drive mechanism.
A need exists in the automotive industry for a drive mechanism that splits torque at the high-speed transmission output between the front wheels and rear wheels before a speed reduction of the transmission output occurs.